It is well known to vary the pitch of the blades on an aircraft propeller, for example in order to maintain the rotational speed of the propeller within close limits. This in turn provides improved performance over a wide range of flight conditions, and enables thrust reversal during landing and ground maneuvering of the aircraft.
A conventional unison ring 10 is formed as an annular ring having a ‘U’-shaped axial cross-section 50 as shown in FIG. 1. The ‘U’-shaped cross-section 50 is formed from a first side member 20 and a second side member 30 that are respectively connected to opposing ends of a floor member 40.
This ‘U’-shaped channel 50 accommodates the lever arms 62 that operate the propeller pitch change spindles 64. Each lever arm 62 comprises a lever arm peg 60 that is accommodated within the “U”-shaped cross-section 50. Each of the pitch change spindles is connected to a corresponding propeller blade (not shown). A pitch change actuator is used to produce axial movement 70 of the unison ring 10, and corresponding axial movement 72 of the lever arm peg 60, which in turn causes rotational movement of the lever arms 62 connected to the pitch change spindles 64, and thus pitch change of the propeller blades.
The unison ring acts as a common point of load transference between the pitch change actuator and the individual propeller blades. As such it is a critical part of the propeller pitch change mechanism.
It also importantly allows for the feathering of the propeller in the event of an engine shutdown in flight. When an engine is shutdown in flight, an unfeathered propeller presents a large and flat surface to the oncoming airflow. This will cause a large drag force on the aircraft which can result in a loss of control of the aircraft.
While it is known to use a pitch lock mechanism to force an unfeathered propeller into a safe feathered position, such mechanisms are complex and heavy, particularly when implemented in an aircraft powerplant having twin contra-rotating propellers.
Even where a pitch lock mechanism is provided, a failure in the unison ring assembly will generally result in the aircraft mission being aborted.
It is desirable to provide a unison ring assembly for a contra-rotating propeller system that provides improved resistance to the initiation and propagation of cracks.
Statements of Invention
According to a first aspect of the present invention there is provided a unison ring assembly comprising:                a plurality of lever arms;        a plurality of pitch change spindles;        a planar backing plate; and        a plurality of pocket-forming portions;         wherein the backing plate is formed as an annular disc, the annular disc being planar in a radial plane of the unison ring assembly, the plurality of pocket-forming portions are equi-spaced circumferentially around the axis of the unison ring assembly, and each of the plurality of pocket-forming portions is attached to the backing plate to form a corresponding radially outwardly opening pocket, with each picket accommodating a respective one of the lever arms, and each lever arm being operable connected to a respective one of the pitch change spindles.        
The use of separate pocket-forming portions which are then attached to the backing plate makes the assembly less susceptible to cracks forming at the edges of the resulting pockets.
The separate pocket-forming portions can be individually attached to the backing plate making the assembly of the unison ring simpler and more convenient for a user.
Optionally, each of the plurality of pocket-forming portions is ‘C’-shaped in circumferential cross-section.
The resulting ‘C’-shaped pocket conformally accepts the drive pin for the corresponding propeller blade pitch adjustment. This ensures that the transfer of loads from the unison ring assembly to the drive pin is direct thus making the unison ring assembly less susceptible to cracking.
Optionally, the plurality of pocket-forming portions is removably attached to the backing plate.
This enables the unison ring assembly to be dismantled for service and repair, thus making the assembly more convenient and cost effective for a user.
Optionally, the plurality of pocket-forming portions is formed integrally.
By forming the pocket-forming portions as an integral part the weight of the unison ring assembly may be reduced which makes it more suitable for weight critical applications.
Optionally, the backing plate comprises a plurality of backing plate portions, each of the plurality of backing plate portions corresponding to a respective one of the pocket-forming portions.
The use of separate backing plate portions for each of the pockets in the unison ring assembly ensures that the point of closure for any one of the pockets is isolated from that for any other one of the pockets. This prevents any cracks initiated at one of the pocket-forming portions from spreading to an adjacent pocket-forming portion.
Optionally, each of the backing plate portions is removably attached to the plurality of pocket-forming portions.
This enables the unison ring assembly to be dismantled for service and repair, thus making the assembly more convenient and cost effective for a user.
Other aspects of the invention provide devices, methods and systems which include and/or implement some or all of the actions described herein. The illustrative aspects of the invention are designed to solve one or more of the problems herein described and/or one or more other problems not discussed.
It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings.